Dura mater is the outermost membrane that comprises the meningeal membrane between the cranial bones and brain. Its function is to protect the brain and inhibit leakage of cerebrospinal fluid. Dura mater also covers the spinal cord providing a similar function. In the field of neurosurgery, any defect or contracture of the dura mater has to be repaired. Various materials have been used for this purpose.
Human-derived dura mater allograft from donors was the most utilized material for repairing the dura mater. However, the incidence of transmission of the agent that causes Creutzfeldt-Jakob disease (CJD) through the use of the human cadaver dura mater has greatly reduced its use. In addition, human dura mater allograft has several disadvantages including low homogeneity and a limited supply.
Other materials have been used as dura substitutes including synthetic polymers such as silicone or ePTFE (expanded polytetrafluoroethylene) and naturally occurring polymers such as collagen. Each of these materials has its own drawbacks. For example, silicone has been reported to predispose patients to meningorrhagia. Silicone acts as a chronic stimulant for the surrounding tissue causing hypertrophy of the granulation tissue. Similar non-biodegradable synthetic materials, such as ePTFE, have been developed but have not become popular because a similar chronic foreign body response develops with their use and their handling characteristics are poor. Other synthetic materials used in bio-absorbable sutures such as polylactides and polyglycolides (PLA/PGA) as described in U.S. Pat. No. 5,861,034 have been developed. However, these materials have yet to show improved clinical performance over earlier unsuccessful synthetic predecessors.
Various naturally occurring biodegradable materials have been investigated. For example, gelatin was one of the earliest biomaterials investigated. Gelatin did not gain acceptance because it had insufficient suture strength for integral use with the internal dura mater. Similarly, other collagenous biomaterials that also lack the necessary suture strength have been described in U.S. Pat. No. 5,997,895. These materials have been commercially available and have gained widespread use as onlay materials. However, when hydrated, these materials convert to a gelatinous mass that has no structural integrity. Thus, there is still a need for a dura substitute that possesses the desirable physical properties such as strength and conformability and in some cases, suturability, and one that has minimal foreign body response.
Microbial cellulose has been suggested for some medical uses. For example, the use of microbial derived cellulose in the medical industry as liquid loaded pads (U.S. Pat. No. 4,788,146), wound dressings (U.S. Pat. No. 5,846,213) and other topical applications (U.S. Pat. No. 4,912,049). Mello et al., (Mello, L. R., et al., Duraplasty with Biosynthetic Cellulose: An Experimental Study. Journal of Neurosurgery, V. 86, 143-150 (1997)) published the use of biosynthetic cellulose similar to the one described in (U.S. Pat. No. 4,912,049) as a duraplasty material in an experimental animal study. Their results showed that the dried form of the microbial derived cellulose was adequate as a dural substitute. However, the material described by Mello et al. does not undergo a depyrogenation step and the material is fully dried while being stretched as described in U.S. Pat. No. 4,912,049.
In contrast, the instant invention provides a non-pyrogenic implantable material and uses either a mechanical or a thermal dehydration method to partially dehydrate the surgical mesh without drying, resulting in two materials that have unique characteristics. Further, these materials can be dried using a supercritical carbon dioxide technology so as to provide a dried product. This processing endows the invention with superior conformability and absorption properties not available in previously described cellulosic materials including the air-dried cellulose of Mello et al.